1. Field of the Invention
The present invention relates to a substrate for an information recording medium used as a recording medium in an information processing device, to an information recording medium that makes use of this substrate, and to a method for controlling the surface of a substrate for an information recording medium.
2. Description of the Related Art
Magnetic disks are known as one type of information recording medium. A magnetic disk is produced by forming a thin film of a magnetic layer or the like on a substrate, and aluminum substrates and glass substrates have been used for this purpose. Recently, however, in response to the need for greater recording density, glass substrates have gradually been edging out aluminum substrates because the former allows for a narrower gap between the magnetic head and the magnetic disk than the latter.
The manufacture of a glass substrate generally involves chemical reinforcement in order to increase the strength of the substrate so that it can withstand the impacts to which it is subjected when installed in a magnetic disk drive. Also being manufactured are crystallized glass substrates whose strength is increased by heat treating and crystallizing the glass substrate surface. Higher recording density is also achieved by polishing the surface of a glass substrate to a high degree of precision so that the flying height of a magnetic head can be kept to a minimum.
The move toward higher recording density is being accomplished not only with glass substrates, but also by making a transition in magnetic heads from thin film heads to magneto-resistive heads (MR heads) and giant (large) magneto-resistive heads (GMR heads).
However, no matter how much the surface roughness (Rmax maximum height) and Ra (center-line mean roughness)) are reduced by precision polishing, the problem was that the flying height of the magnetic head could not be lowered. It was found that a reduction of flying height of magnetic head could not be achieved unless surface roughness and microwaviness of the substrate surface were within a specific range and satisfied a specific relationship.
The inventors discovered that the roughness and microwaviness of a substrate surface are closely related to glide height (touchdown height), and it is an object of the present invention to provide a substrate for an information recording medium, and the information recording medium itself, with which the desired glide height (touchdown height) can be achieved by setting the roughness and microwaviness of a substrate surface within a specific range and relationship.
The present invention was conceived in light of the above object, and is constituted as follows.
Constitution 1
A substrate for an information recording medium, wherein the period of microwaviness is 2 xcexcm to 4 mm, and if we let wa be the maximum height of this microwaviness and Rmax be the maximum height measured by atomic force microscope, the main surface of the substrate has a wa of no more than 5 nm and an Rmax of no more than 12 nm, provided that wa is the difference between the highest and lowest points on a measurement curve of all measured points in a measurement area.
Constitution 2
A substrate for an information recording medium, wherein the period of microwaviness is 2 xcexcm to 4 mm, and if we let wa be the maximum height of this microwaviness and Rmax be the maximum height measured by atomic force microscope, when wa is expressed by x and Rmax by y, the following are satisfied: xxe2x89xa65 nm, yxe2x89xa612 nm, yxe2x89xa7(10/3)xxe2x88x9210, yxe2x89xa6(10/3)x+2, provided that wa and Rmax are defined the same as in Constitution 1.
Constitution 3
A substrate for an information recording medium, wherein the period of microwaviness is 2 xcexcm to 4 mm, the product of multiplying the maximum height wa (units: nm) of this microwaviness by the maximum height Rmax (units: nm) measured by atomic force microscope is compared with the result of forming at least a magnetic layer on the main surface of the substrate for an information recording medium and conducting a touchdown height test, and a specific Rmaxxc3x97wa is set so that the desired touchdown height value will be obtained for the correlation between the product of Rmax and wa and the touchdown height(TDH), provided that wa and Rmas are defined the same as in Constitution 1.
Constitution 4
A substrate for an information recording medium, wherein the period of microwaviness is 2 xcexcm to 4 mm, and if we let wa (units: nm) be the maximum height of this microwaviness and Rmax (units: nm) be the maximum height measured by atomic force microscope, the following is satisfied: Rmaxxc3x97waxe2x89xa658(nm*nm), provided that wa and Rmas are defined the same as in Constitution 1.
Constitution 5
The substrate for an information recording medium according to any of Constitutions 1 to 4, wherein wa is a value obtained by excluding any abnormal protrusion points from the measured points.
Constitution 6
The substrate for an information recording medium according to any of Constitutions 1 to 5, wherein the substrate is a glass substrate.
Constitution 7
The substrate for an information recording medium according to any of Constitutions 1 to 6, wherein the substrate is a substrate for a magnetic disk.
Constitution 8
An information recording medium, wherein at least a recording layer is formed on a glass substrate for an information recording medium according to any of Constitutions 1 to 7.
Constitution 9
The information recording medium according to Constitution 8, wherein the recording layer is a magentic layer.
Constitution 10
A method for controlling the surface of a substrate for an information recording medium, wherein the period of microwaviness measured for the main surface of the substrate for an information recording medium is 2 xcexcm to 4 mm, the correlation is found between the product of the maximum height wa (units: nm) of this microwaviness and the maximum height Rmax (units: nm) measured by atomic force microscope and the touchdown height T of said information recording medium when at least a magnetic layer is formed on the main surface of the substrate for an information recording medium, and the Rmaxxc3x97wa of the substrate for an information recording medium is determined such that the information recording medium will have the desired touchdown height for the found correlation, provided that wa and Rmas are defined the same as in Constitution 1.
Constitution 11
The method for controlling the surface of a substrate for an information recording medium according to Constitution 10, wherein wa is a value obtained by excluding any abnormal protrusion points from the measured points.